Overview

Alaska is approximately one-fifth the area of the 48 contiguous states. This large domain encompasses several distinct climatic and ecological regimes, and includes nearly 80% of U.S. public lands. There are approximately 114 million acres of forest in the northern interior region of Alaska (Fairbanks region), and an additional 12.5 million acres of forest along Alaska's rainy Pacific coastal region (Anchorage and southeast Alaska region).

Results from several studies at the Bonanza Creek Long-Term Ecological Research (LTER) site, as well as research results from elsewhere in Alaska, provide unambiguous evidence of a significant climate warming over the past century. Observational records of climate change dating back to the early 20th century are currently available for just a few localities in Alaska. However, tree-rings and other proxy indicators of climate change are being used to supplement the instrumental records and to help reconstruct climate trends in Alaska prior to the 20th century.

These climate records indicate that Alaska experienced a major shift toward warmer conditions in the late 1970s, as did much of the world. As a result, the climate in the northern interior (Fairbanks region) and coastal forest regions (Anchorage region) of Alaska is now warmer than at any time in the last several centuries and very probably the last millennium. The magnitude of this recent shift, as well as the rate of climate warming and projected future changes, pose a host of problems and opportunities for the people of Alaska, quite often having international economic implications as well. Such changes are also impacting the infrastructure presently in place, and may affect the chances of survival of forest and other ecosystems of Alaska, the goods and services of which are crucial to the region.

The observed changes in climate in Alaska are generally consistent with the projections from climate models that have been used to simulate the warming effect of increasing greenhouse gas concentrations in the atmosphere. In particular, the model results project that high-latitude land areas of the Northern Hemisphere would experience the most rapid and largest warming on Earth.

Climate Changes are Affecting
the Forest Ecosystems of Alaska

Interior Alaska - Fairbanks Region

Tree-ring records indicate that northern and central Alaska is warmer now than at any time since the medieval warm interval of about 1000 A.D. The observed mean annual temperature of the northern interior region of Alaska in the vicinity of Fairbanks has increased ~1.4ºC (2.6ºF) during the 20th century. Records indicate that the climate warmed in this region from 1850 to 1940, then cooled from 1941 to 1975, and has warmed significantly since 1976. The warming from the mid-1970s to the mid-1990s averaged 1ºC per decade, totaling ~3ºC over this period. In addition, summer precipitation has decreased at the rate of ~17% per 100 years during the period of record in the Fairbanks area. Snowfall totals at Fairbanks, on the other hand, are up ~60% during the 20th century; substantially greater increases have occurred on the south-facing mountain slopes that intercept moisture-bearing winds from the Pacific. Heavy snow loads appear to be a major factor in tree canopy breakage. Injured pockets of trees attract wood-boring insects that can build up in numbers and, if climate conditions are favorable, cause widespread tree death, with far-reaching ecological and social consequences.

The growth of low-elevation forests in the interior of Alaska is directly limited by summer warmth and moisture deficit, as indicated by an analysis of the physical and chemical properties of tree-rings. The results indicate that the last 20 years have been the least favorable period for the growth of white spruce trees in the 20th century, and almost certainly for the last 400 years. Tree growth in these white spruce forests has been reduced by 50% in some cases, and growth of paper birch has significantly declined as well. There are many new instances of insect infestation of stressed trees, and spruce cone crops at various places have either not occurred or are so low as to not provide the numbers of seeds necessary for effective reproduction. Furthermore, the total area burned by forest fires in Alaska is directly related to summer temperature, and the incidence of fires would be expected to increase as summer temperatures increase.

Coastal South-Central Alaska - Anchorage Region

Mean annual and mean summer temperatures at Anchorage in coastal south-central Alaska have warmed at a rate of ~1.6ºC (~3ºF) per century during the 20th century. Unlike the interior Fairbanks region of Alaska, however, precipitation at Anchorage has increased at a rate of ~23% per century. Accordingly, white spruce trees in the Anchorage and south-central Alaska regions have experienced strongly accelerated growth that is directly correlated with the warmer and moister conditions. However, one of the largest outbreaks of insect-caused (spruce bark beetle) tree mortality in the history of North America has left most trees dead over ~3 million acres of forest land. Spruce bark beetle populations have historically been limited or kept in check by cool summers and cold winters, so rising temperatures (the most likely cause of the beetle outbreak) have not been beneficial in this case. Commercial forest value and many of the non-market values (e.g., recreational value) have been dramatically reduced over most of the region.

Southeastern Alaska

The sitka spruce/western hemlock rainforest of southeast Alaska is highly valued for timber, wildlife and fisheries habitat, and as the setting for rapidly expanding tourism and wilderness recreation. In southeast Alaska, the number of days with gale-force winds have more than doubled since 1950, increasing the risk of extensive tree blowdown. Warmer summer weather and extended rainless intervals have triggered outbreaks of the defoliating western black-headed budworm, and apparently have increased the number and duration of low stream flow episodes; it is these conditions that block the return of spawning salmon and generally limit municipal and industrial water supplies.

Three of the four earliest spring break-ups of ice on the Tanana River have occurred in the 1990s, the warmest decade in the 81-year record.

Even with warmer conditions in winter, temperatures remain below freezing. Thus, as the climate in Alaska has warmed, snowfall has generally increased as the moisture-holding capacity of the atmosphere has increased.

Soil temperature in cold regions is controlled primarily by the mean annual temperature and the insulating effect of snow (depth and duration). The combination of increased annual temperature and increased snowfall in the past 20 years has tended to rapidly increase the temperature of most of the soil and permafrost in central and southern Alaska. Warmer soils speed up the reactions that decompose stored soil organic matter, thus causing a net release of CO2 to the atmosphere that adds to the greenhouse effect.

Model studies and field measurements of soil temperature at the Bonanza Creek LTER site indicate that, for most of the discontinuous permafrost in that region, the seasonally thawed soil layer above the permanently frozen soil layer is penetrating more deeply in recent years - leaving the remaining permanently frozen soil below very close to the thaw point, much of it only a few tenths of a ºC below freezing.

Nearly all permanently frozen soil contains ground ice masses that when thawed lead to surface subsidence. Roads and structures built on permafrost can suffer severe damage from thaw-related subsidence. Most structures have been built to withstand a certain amount of permafrost thawing which always accompanies site disturbance, but the recent warming frequently exceeds structural design limits. Subsidence features in natural environments often collect water from the melting of ice in soils, which greatly speeds up the transfer of heat to the soil. Thawed subsidence sites also often change from black spruce woodland ecosystems to wetland, potentially increasing the release of methane during the decomposition of soil peat. Thaw-related subsidence is becoming more noticeable in central Alaska as well. In fact, extensive areas of Alaska are projected to undergo thaw-related subsidence with only slightly more warming.

Acknowledgments: The data presented in this seminar represent a synthesis of Dr. Juday's own work as well as the work of several investigators at the Bonanza Creek LTER site and the Lamont-Doherty Tree-Ring Laboratory.